1. Field of the Invention
This invention relates to improvements in a device for detecting that the speed of a moving object has reached or crossed a set speed threshold value.
2. Description of the Prior Art
In safety devices for elevators when the speed of an elevator cage reaches a preset speed, the operation of the elevator is suspended to stop the cage. One device for detecting whether or not a set speed threshold value has been reached digitally detects the preset speed by utilizing the combination of a time limit circuit and a detector circuit for analyzing pulses generated at intervals inversely proportional to the speed of the cage.
Such a speed threshold value crossing detecting device has the advantage of having a fast response. However, the detector circuit's output unnecessarily repeats signals as a function of the rate of the input pulses which in turn depends on the machining precision of the disc and the electrical characteristics of the pulse generator, etc. Usually, several devices are controlled by the detector circuit's output, and fluctuations in said output results in poor speed control. For example, in the case of performing a braking control, a brake is repeatedly turned "on" and "off", which does not allow for smooth braking control as well as shortening the lifetime thereof.